Hi all,
I am struggling with a slight offset in a new application we are
developing which is a flash application (global risk mapping) and which
will take in overlaid WMS layers served by MapServer in Winkel Triple. I
have reprojected the base data (shapes, rasters) so that MapServer does
not have to reproject on the fly, and performance seems to be good.
However, I am having a problem when zooming in and new WMS request are
being made. The more you zoom in, the more offset becomes visible
between the base (flash) layer (generated by exporting an ai file of the
same shapefile) and the overlaid MapServer png.
Perhaps this has to do with the spatial extent of my data and the
width/height ratio (SIZE)?! I used ogrinfo shape.shp -al -summary to
generate the extent of my shape, which I then used as the extent of the
map file. For the SIZE parameters I used the width I need for my png and
calculated the height based on the width/height ratio in the above
mentioned extent. Is this the right way to do so?! Or am I missing
something here?
The image looks fine, except when zooming in and new requests are made
using an adjusted bounding box (dimensions divided by a factor), the
offset becomes slightly bigger with each zoom factor, as if a slight
skewing is taking place.
Unlike the case in unprojected (lat/long) data, I cannot change these
parameters much at all (about 2 pixels) before MapServer gives me an
empty image. Is there a way in which MapServer will more flexibly allow
me to change the range of these parameters? I also noticed that WMS
requests give me bitmaps with dimensions consisting of whole integers. A
rounding off takes place. This could result in some potential
inaccuracies along borders too of course.
Some of these problems wouldn't occur if we were just using a MapServer
based mapping application, however we do need to combine and line it up
with the flash layer for other purposes.
I will now play around with the limited range in which I can change
these parameters, but maybe I am missing a more obvious thing?!
Any help/advice on this issue would be greatly appreciated! I have
included part of the map file below.
Regards,
Mark de Blois
Maplecroft
#
#Start of map file, created by AmeiN! for ArcGis 13/09/2006, 15:20:54
#
MAP
NAME 'MaplecroftWMS'
STATUS ON
PROJECTION
"proj=wintri"
"ellps=WGS84"
"lat_1=45"
END
SIZE 697.8 401.31
EXTENT -16707480.310734 -10018754.171395 16920780.652974 9320947.177797
UNITS meters
LAYER
NAME 'Countries_WT'
DATA './admin/maplecroft_countries_winkel_45'
STATUS off
TYPE Polygon
TOLERANCE 7
TOLERANCEUNITS pixels
METADATA
WMS_TITLE 'Countries_WT'
WMS_FEATURE_INFO_MIME_TYPE 'text/html'
END #METADATA
PROJECTION
"proj=wintri"
"ellps=WGS84"
"lat_1=45"
END
CLASS
NAME 'Countries'
STYLE
#COLOR 255 255 212
OUTLINECOLOR 255 0 0
END #STYLE
END #CLASS
TEMPLATE "winkel_query.html"
END #LAYER
